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Old 10th Nov 2018, 19:38
  #967 (permalink)  
Join Date: Jul 2014
Location: Germany
Posts: 344
Originally Posted by Gysbreght View Post
Secondly, I wonder what really occured on previous flights. Was it really an UAS situation or were above alerts incorrectly diagnosed as a pitot-static system problem?
Looking at the altitude profile the accident and previous flight there is this sudden descent.
That fits with a high reading AoA sensor leading to stick shaker activation followed by a pitch down by the pilot.
Also the automatic trim down would have activated.

Now i've been told before on this thread only the AoA can trigger the stick shaker which makes not much sense to me (the posts were deleted anyway).
My understanding is the combination of AoA and airspeed leads to stick shaker activation.
If someone wants to explain the exact stick shaker logic on the 737-8 please feel free i would be curious

Anyway the plane displayed IAS disagree with the stick shaker activating on the captains side. They continued the previous flight using the instrumentation on the right hand side.

The AoA sensor was replaced according to KNKT, see this quote from avherald:
On Nov 8th 2018 the KNKT reported an angle of attack sensor had been replaced on Oct 28th 2018 following the flight JT-775 from Manado to Denpasar (the aircraft completed the subsequent flight JT-43 to Jakarta and suffered the crash the next flight JT-610). The aircraft subsequently flew to Jakarta, the crew however reported there were still problems. The search for the CVR is hampered by thick mud.
So maybe they introduced the AoA problem by replacing the sensor or they made it worse, the sensor that came off the airplane is at Boeing being tested.
It's also possible that the plane already had an underlying pitot-static issue that was not fixed.
Or somehow the input logic receiving the AoA data was flawed and replacing the sensor did nothing.

Without more facts the answer to your question is mostly just: Outside the investigation no one can say for sure at the moment.
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